For instance, a gas sensor, which outputs an electromotive force, is provided at a vehicle engine. In this type of gas sensor, exhaust gas, which is discharged from the engine, serves as a sensing subject of the gas sensor, and an oxygen concentration of the exhaust gas is sensed with the gas sensor. This type of gas sensor includes an electromotive force cell, which outputs an electromotive force signal that varies depending on whether the exhaust gas is rich or lean. Specifically, when an air-to-fuel ratio is rich, the electromotive force cell outputs the electromotive force signal of about 0.9 V. In contrast, when the air-to-fuel ratio is lean, the electromotive force cell outputs the electromotive force signal of about 0 V.
In this type of gas sensor, when the air-to-fuel ratio of the exhaust gas changes between rich and lean, a change in the sensor output may be disadvantageously delayed relative to an actual change in the air-to-fuel ratio. In order to improve the output characteristic of such a gas sensor, various techniques have been proposed.
For instance, the Patent Literature 1 discloses a gas sensor control device, in which a constant current circuit is connected to at least one of a pair of sensor electrodes. In this gas sensor control device, when it is determined that a demand for changing the output characteristic of the gas sensor is present, a flow direction of the constant electric current is determined based on the demand. Then, the constant current circuit is controlled to induce a flow of the constant electric current in the determined direction. Through the supply of the constant electric current, the output characteristic of the gas sensor is appropriately controlled.
The air-to-fuel ratio control operation, which uses the sensed value of the gas sensor, significantly contributes to the reduction of the exhaust emissions of the engine. In a case where an abnormality is present in the gas sensor, the abnormality has a significant influence with respect to the reduction of the exhaust emissions. Furthermore, even in a case where the gas sensor is normal, when the constant electric current does not normally flow in the electromotive force cell, it may possibly have an influence on the exhaust emissions. With respect to the above point, it is desirable to correctly determine whether an abnormality is present in the constant current circuit.